Underwater power plants which absorb kinetic energy in a free-standing manner without any dam structure from a water flow are especially suitable for power generation from an ocean current, preferably a tidal current. An advantageous embodiment for underwater power plants comprises propeller-shaped water turbines with a plurality of rotor blades-which are fixed to a revolving unit.
Such a water turbine can be fastened to a support structure via a machine gondola in which an electric generator is typically accommodated, which support structure is arranged to be fixed to foundations in the floor of the water body or to be floating. For the economic design of generic underwater power plants, especially for power generation from a tidal current, the mean flow speeds that occur over the course of the year are typically used as a basis. However, in the case of a location of the plant in the sea, inflow speeds may occur in certain cases at least over short periods of time which lie above the chosen plant configuration. Such extreme situations occur especially under stormy conditions, for which the wind and waves run in the direction of the tidal current. The mechanical power absorbed by the water turbine from the flow needs to be limited in such cases of stress.
A known form of power limitation provides the use of an active blade angle adjusting device. This allows choosing the angle of attack of the rotor blades of the driving flow in such a way that a desired power curve is obtained. In extreme situations, the rotor blades are guided in the direction of the feathered pitch position and the power intake is thus limited. Accordingly, the components downstream of the water turbine are protected in the drive train against overloading. The disadvantageous aspect in this approach is the constructional effort required for arranging such a blade angle adjusting device. Moreover, additional movable components are necessary in the revolving unit which lead to an increased failure risk and consequently require regular maintenance.
In order to achieve a sturdy configuration of the system that requires as little maintenance as possible, speed guidance by means of the supporting effect through the electric generator in the drive train can be caused as an alternative measure for power limitation. The generator and possibly further braking apparatuses will reduce the speed of the rotor in the case of an overload in order to guide the same away from power optimum. This reduction in speed can go so far that a stall occurs at the profile of the rotor blades, so that power intake is reduced efficiently. The disadvantageous aspect in this approach is that mechanical loads occurring under the conditions of a stall are high as a result of the occurring blade excitations and require a respective configuration of the structural stiffness of the rotor blades. A high constructional effort is the result of this requirement placed on strength and makes the water turbine heavier.
When the above speed guidance of the water turbine for power limitation is arranged in such a way that in the case of overload an increase of speed to a rotational speed above the best point is allowed, it is possible to prevent stalling. However, high forces will act on the water turbine in the range of high rotational speeds. The increasing centrifugal moments need to be taken into account for the design of the plant especially in the range of high speeds.